In general, in a mold type plastic gear having teeth along its circumference, a portion of its wall thickness must be removed when face width of said gear becomes over 2 mm. This is carried out for assuring the accuracy of gear. However, a simple wall-removed shape of a gear brings about a decrease of its rigidity and an increase of in the amount of deformation of the gear during meshing action, thereby reducing the gear performance
For assuring the rigidity of mold type plastic gear which has undergone a thickness-removal operation, it is general to have some radial ribs connected to tooth root of teeth between hub of gear and said tooth root of teeth. However, shrinkage cavity of resin occurs at the connection between said tooth root and said ribs when forming the gear, thereby to considerably reduce the roundness of the gear.
Therefore, in place of connecting said ribs directly to said tooth root of teeth, another gear construction is proposed wherein a circular rim is formed at the distance from and inside of said tooth root and radial ribs are formed on the inside of said rim (refer to Japanese Patent Laid Open No. 96-25501). In such a gear construction, even if said ribs are connected to said rim, local shrinkage cavity of resin at teeth portion of gear can be prevented, thereby to assure rigidity of gear while maintaining its accuracy.
On the other hand, it is important that pressure against the gear, particularly against the teeth portion thereof is uniformly maintained until the last step of the forming process wherein the resin is cooled and solidified. When the circular rim is formed at a distance from tooth root, the gates are preferably located in the web portion on the inside of said rim. However, in this case, since said rim is located in the flowing route of the resin from said gates to said teeth portion, the cooling speed of the resin at respective portions of gear is affected and it becomes difficult to maintain uniform pressure particularly against the teeth portion of gear until the last step of forming operation.
Further, in the case of a composite gear having an outer teeth portion along its circumference as well as an inner teeth portion or mechanism such as cam around said hub, it is necessary to assure the rigidity of said outer teeth portion having a larger diameter and also to maintain pressure against said inner teeth portion or said mechanism around said hub which need a high accuracy until the last step of forming operation.